This chapter reviews the role of endometrial and subendometrial blood flow determined by Doppler ultrasound in the prediction of pregnancy during in vitro fertilization (IVF) treatment. Uterine Doppler study may not reflect the actual blood flow to the endometrium as the major compartment of the uterus is the myometrium and there is collateral circulation between uterine and ovarian vessels. Absent endometrial and subendometrial blood flow has been shown to be associated with no pregnancy or a significantly lower pregnancy rate. In combination with a 3D ultrasound, power Doppler provides a unique tool with which to measure the blood flow towards the whole endometrium and the subendometrial region. There was a significant elevation in the middle to late follicular phase, followed by a substantial fall and a secondary slow luteal phase rise that was maintained until the onset of menstruation. Doppler flow study of spiral arteries is not predictive of pregnancy.